1. Field of the Invention
The present invention is related to a method and apparatus for converting power. More specifically, the present invention is related to a method and apparatus for converting power with an AC to DC converter.
2. Description of Related Art
The prior art has shown that it is possible to provide power to remote devices using Radio-Frequency (RF) electromagnetic waves. Wireless power transfer has been described in great detail by W. C. Brown in U.S. Pat. No. 3,114,517, “Microwave Operated Space Vehicles,” incorporated by reference herein, and within numerous other articles by the stated author. Wireless power transfer is also used to provide power to Radio-Frequency Identification (RFID) tags. The transmitted RF power is captured by an antenna and rectified using a number of disclosed circuits to provide Direct Current (DC) to a load. U.S. Pat. No. 3,434,678, “Microwave to DC Converter,” incorporated by reference herein, describes an apparatus for converting microwave power to DC using the bridge rectifying circuit shown in FIG. 1.
More recent patents such as U.S. Pat. No. 6,140,924, “Rectifying Antenna Circuit,” and U.S. Pat. No. 6,615,074, “Apparatus for Energizing a Remote Station and Related Method,” both incorporated by reference herein, describe RF to DC converters that are implemented using voltage doubling rectifier configurations as shown in FIG. 2.
The function of these circuits is acceptable when the input power and the load impedance are constant. However, variations in either the input power or load impedance degrade the overall conversion efficiency of the circuit. The conversion efficiency is defined as the rectified output DC power divided by the Alternating Current (AC) power input to the rectifier. Examples of how changes in the load resistance (or equivalent resistance) and input power affect the conversion efficiency are shown in FIGS. 3 and 4, respectively.
Changes in the rectifier conversion efficiency for varying input power and output load were described in U.S. Pat. No. 6,212,431, “Power Transfer Circuit for Implanted Devices,” incorporated by reference herein, which teaches in Column 1 lines 55-62 that when transferring power inductively from an external coil to an implanted device that “Unfortunately, neither the load associated with the implant device nor the separation distance between the external coil and the implant coil are constants. Each of these parameters are, in practice, variables, that may vary, e.g., from 3-to-15 mm for the separation distance, and 20 to 300 ohms for the load. As a result, optimum power transfer between the external device and implant device is rarely achieved. Thus, a less than optimum power transfer condition exists . . . . ” In this quotation, the separation distance is analogous to changing the input power to the implanted device. The solution proposed in U.S. Pat. No. 6,212,431 is to vary a matching parameter on the external transmitting coil to optimize the power transfer from the external transmitting coil to the implanted receiving coil. The invention disclosed in U.S. Pat. No. 6,212,431 implements the solution at the transmitter, which limits the system to one receiver because the transmitter must vary its output based on a single receiver. Also, U.S. Pat. No. 6,212,431 makes no mention of a rectifying circuit and the effect this may have on the method and apparatus presented. Additionally, U.S. Pat. No. 6,212,431 relies on inductive coupling, which allows the impedance of the implanted device to be seen by the transmitting coil in a similar manner of reflecting the impedance on the secondary side of a transformer to the primary side. The invention described herein does not rely solely on inductive or near-field power transfer, but rather includes operation in the far-field where reflecting the receiving load to the transmitting side is not possible.
Varying load impedances are also examined in U.S. Pat. No. 6,794,951, incorporated by reference herein, which describes a transmitting circuit to ionize gas to create a plasma. The problem presented is that the load seen by the transmitter changes depending on the status of the plasma in the chamber. When no plasma is present, the transmitter sees a certain impedance value. However, when there is plasma present in the chamber, a different impedance value is seen by the transmitter. To combat this issue, U.S. Pat. No. 6,794,951 proposes a dual impedance matching circuit, which is controlled via a switch selection system. During the start mode, the first impedance matching circuit is used to match when no plasma is present in the chamber. During the run mode, the second impedance matching circuit is used to match the system with plasma in the chamber. The solution presents a way to drive discrete load values on an RF transmitter. This solution is limited to the transmitting side, must know the discrete impedance values seen during the multiple modes in order to design the impedance matching networks, must have active switching to control the matching network, and is designed to give an RF output.